Pokemon Platinum is the next part of Pokemon Diamond and Pokemon Pearl. Pokemon Platinum was planned to be released on March 22, 2009. From the preliminary details that I read and saw and heard, the game is going to feature a new story and some new Pokemon shapes, sizes etc.
Basically this concept of RPG games has become an industry standard and Pokemon is no different. The idea is for the player to log into the repository and explore the world in order to collect beings that are Pokemon.
Each Pokemon has a special power and strengths which make it attractive etc.
Pokemon Platinum is supposed to take place in the new world especially created for this series. The world is called Distortion World and the key features of this world are that the normal rules of engagement like time and space do not exist in this. In this world things are topsy turvy and regular world laws of physics do not apply. (So you could actually walk on the wall or upside down etc).
Some of the new characters that I am aware of being introduced in this game are Charon and Looker. These are funny stupid names but hey the fans like them so no complaints.